1. Technical Field
A system concerns acquiring a B1 map reflecting information regarding a B1 magnetic field formed by a combination of two or more of a plurality of RF coil elements.
2. Description of the Related Art
A known magnetic resonance imaging (MRI) system provides non-invasive imaging, improved contrast of tissues compared to a computer tomography (CT) element, and is free of bone tissue related artifacts. Furthermore, a magnetic resonance imaging (MRI) system is capable of capturing various tomography images in desired directions without relocating a target object, thus being widely used. A known MRI system generates a magnetic resonance (MR) image using differences between characteristics of tissues of a target object that reflects differences between magnetic resonance characteristics of tissues of a target object. Uniformity of a magnetic field (B1) generated by radio frequency (RF) pulses in an MRI system affects uniformity of an MR image. Therefore, a B1 shimming process is employed to improve B1 uniformity. The B1 shimming process may include driving respective coil elements included in a transmission RF coil unit having separate circuit structures by driving a plurality of coil elements with signals having different magnitudes and phases. To perform the B1 shimming, it is necessary to determine spatial B1 magnetic field distribution (B1 mapping).
A B1 magnetic field generated by RF pulses may be separated into a transmission RF magnetic field (B1+ magnetic field) component rotating in a clockwise direction and a reception RF magnetic field (B1− magnetic field) rotating in a counterclockwise direction. Here, the B1+ magnetic field induces nucleomagnetic resonance with respect to a magnetization vector, thereby laying the magnetization vector on a transverse plane. When a magnetization vector lies on a transverse plane, the magnetization vector rotates on the transverse plane at the Larmor frequency, and the rotation of the magnetization vector induces an electromotive force (EMF) in a reception RF coil. MR signals received by the reception RF coil are affected by the B1− formed by the reception RF coil. In an MRI system, a B1+ magnetic field formed by a transmission RF coil contributes to formation of MR signals in an RF signal transmission mode, whereas a B1− magnetic field formed by a reception RF coil contributes to formation of MR signals in an RF signal reception mode.
A transmission RF magnetic field (B1+ magnetic field) is a magnetic field which induces actual magnetic resonance by rotating a magnetization vector of at least one type of atomic nucleus included in a target object in a main magnetic field direction when RF pulses are applied to the target object via an RF coil in an MRI system. Therefore, in an MRI system, it is necessary to suppress formation of a B1− magnetic field and strengthen a B1+ magnetic field as much as possible to acquire a high-quality MR image. Furthermore, to form a uniform B1+ magnetic field, it is necessary to precisely measure magnitude and spatial phase distribution of the B1+ magnetic field.